1. Field of the Invention
This invention relates to X-ray examination apparatus in which an X-ray filter is provided between an X-ray source and an X-ray detector. An object to be examined, for example a patient, is positioned between the X-ray filter and the detector.
2. Description of Related Art
Apparatus of this type is described in WO 97/03450.
The filter used in such an apparatus enables an X-ray image to be obtained with a low dynamic range. This is desirable so that small details of the X-ray image, which occur as small variations in contrast, may be analysed. This is possible by using the filter to block signals which would give rise to a larger dynamic range. Some parts of the body, for example lung tissue, have very high X-ray transmittance whereas other parts of the body, for example bone tissue, can hardly be penetrated by X-rays. Without filtering, this difference gives rise to the large dynamic range. The reduction in dynamic range achievable using a two-dimensionally controllable X-ray filter simplifies image intensifying techniques for the detected image. The ability to shield certain areas of the body using the filter may also be desirable to protect certain types of body tissue from X-ray exposure.
The filter described in WO 97/03450 comprises an array of capillary tubes, each containing an X-ray absorbing liquid, and the height of the liquid in each tube is controllable to provide a different degree of X-ray absorption. WO 97/03450 describes a method of controlling the level of liquid in each capillary by applying a given fixed voltage signal over a different time period for different capillaries. The change in liquid level as a function of time is used as a control parameter. The capillaries are arranged in rows and columns, and the capillaries are addressed row by row. It has also been proposed that the filter adjustment should be divided into a number of time frames so that the liquid level of each row may be adjusted in partial stages. This has been considered desirable because the physical inertia of the liquid is considered to favour liquid movement in discrete steps. Furthermore, a smaller time difference then exists between the filling of capillaries in the first rows and those in the last rows. This may enable imaging to be started during the latter stages of the filter adjustment period
A problem which is not addressed by WO 97/03450 is the need to reduce the overall time required to adjust the X-ray filter from one configuration to a new configuration. The time required to change the liquid level in an individual filter element is not only a function of the physical inertia of the X-ray absorbing liquid, but also is a function of the effective electrical capacitance of the filter elements, which varies as the liquid level varies.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant's invention of the invention subsequently claimed.